1. Field of the Invention
The present invention relates to a new and improved fluidized bed steam generating system and more particularly to a system wherein a waterwall structure is provided to enclose an upstanding combustion vessel on one side and a convection pass boiler and heat exchanger on the other side of a common central waterwall.
2. Description of the Prior Art
U.S. Pat. Nos. 4,084,545 and 4,154,581 disclose fluidized bed systems wherein a reactor vessel is provided with a dense bed section including large size or coarse solid particulate material maintained in a fluidized condition while a flow of fine solid particulate material in a highly fluidized state is recirculated through the vessel between a gas/solids separator and an external heat exchanger. Hot gaseous products of combustion are taken from the gas/solids separator and utilized for heating boiler tubes in a convection pass boiler to generate steam. Problems have developed in systems of the type described in the aforementioned patents because of the separation and spaced-apart location of the various major components of the system. Also, the use of refractory materials has greatly impacted on component weights, and has affected maintenance procedures and schedules for completing and maintaining an operative system, causing it to be generally desirable to minimize the amount of refractory material used. Another problem associated with systems of the type described is the problem of differential thermal movement between component parts, in particular where refractory lined flow conduits are provided. Moreover, long or crooked horizontally extending conduits for recycling the fine solid particulates have presented operational problems resulting in uneven distribution and flow in the reactor vessel of the system. Poor air distribution and gaseous interaction with entrained solids has caused combustion and temperature distribution problems and these problems have resulted in reduced steam output capacities and reduced thermal heat transfer efficiency. Other problems have occurred in external heat exchangers having sparger type fluidized bed air distributors and fuel feeding problems have also been encountered. Sometimes pneumatic lift lines have been employed and these have incurred problems from both a mechanical and a physical arrangement standpoint and problems with internal flow distribution due to interactions between the transport air flow and the recycled fine solid materials have also occurred.